The U.S. ambassador to the United Nations says the United States is trying to establish what really happened before it considers a response to allegations poison gas was used again in Syria.

Ambassador Samantha Power said Sunday reports of poison gas use are "unsubstantiated," but the United States will do everything in its "power to establish what has happened and then consider possible steps in response."

Both sides in Syria's civil war blamed each other for allegations gas was used in attacks Friday and Saturday, during the ongoing international effort to rid the country of chemical weapons.

Also Sunday, Syrian government forces launched air raids on rebel-held areas around the capital, Damascus, and districts of Aleppo, with dozens of deaths and injuries reported.

Meanwhile, in Damascus, Syrian President Bashar Assad said the conflict is shifting in the government's favor.

State-run Syrian television quoted the president as telling students and teachers from Damascus University, "This is a turning point in the crisis, both militarily in terms of the army's continuous achievements in the war against terror or socially in terms of national reconciliation and growing awareness of the true aims of the attack on the country. "